Isolation and mRNA expression analysis of aquaporin isoforms in marine medaka Oryzias dancena, a euryhaline teleost

Salinity effects on expression and localization of aquaporin 3 in gills of the euryhaline milkfish (Chanos chanos)

Milkfish (Chanos chanos) are important euryhaline fish in Southeast Asian countries that can tolerate a wide range of salinity changes. Previous studies have revealed that milkfish have strong ion regulation and survival abilities under osmotic stress. In addition to ion regulation, water homeostasis in euryhaline teleosts is important during environmental salinity shifts. Aquaporins (AQP) are vital water channels in fish, and different AQPs can transport water influx or outflux from the body. AQP3 is one of the AQP channels, and the function of AQP3 in the gills of euryhaline milkfish is still unknown. The aim of this study was to investigate the expression and localization of AQP3 in the gills of euryhaline milkfish to contribute to our understanding of the physiological role and localization of AQP3 in fish. The AQP3 sequence was found in the milkfish next-generation sequencing (NGS) database and is mainly distributed in the gills of freshwater (FW)-acclimated milkfish. Under hypoosmotic and hyperosmotic stress, the osmolality of milkfish immediately shifted, similar to the aqp3 gene expression. Moreover, the abundance of AQP3 protein significantly decreased 3 h after transferring milkfish from FW to seawater (SW). However, there was no change within 7 days when the milkfish experienced hypoosmotic stress. Moreover, double immunofluorescence staining of milkfish gills showed that AQP3 colocalized with Na+ /K+ ATPase at the basolateral membrane of ionocytes. These results combined indicate that milkfish have a strong osmoregulation ability under acute osmotic stress because of the quick shift in the gene and protein expression of AQP3 in their gills.

Multiple Instances of Adaptive Evolution in Aquaporins of Amphibious Fishes

Aquaporins (AQPs) are a highly diverse family of transmembrane proteins involved in osmotic regulation that played an important role in the conquest of land by tetrapods. However, little is known about their possible implication in the acquisition of an amphibious lifestyle in actinopterygian fishes. Herein, we investigated the molecular evolution of AQPs in 22 amphibious actinopterygian fishes by assembling a comprehensive dataset that was used to (1) catalogue AQP paralog members and classes; (2) determine the gene family birth and death process; (3) test for positive selection in a phylogenetic framework; and (4) reconstruct structural protein models. We found evidence of adaptive evolution in 21 AQPs belonging to 5 different classes. Almost half of the tree branches and protein sites that were under positive selection were found in the AQP11 class. The detected sequence changes indicate modifications in molecular function and/or structure, which could be related to adaptation to an amphibious lifestyle. AQP11 orthologues appear to be the most promising candidates to have facilitated the processes of the water-to-land transition in amphibious fishes. Additionally, the signature of positive selection found in the AQP11b stem branch of the Gobiidae clade suggests a possible case of exaptation in this clade.

Differential expression of aquaporin genes and the influence of environmental hypertonicity on their expression in juveniles of air-breathing stinging catfish (Heteropneustes fossilis)

Aquaporins (AQPs) are a superfamily of transmembrane channel proteins that are responsible for the transport of water and some other molecules to and from the cell, mainly for osmoregulation under anisotonicity. We investigated here the expression patterns of different AQP isoforms and also during exposure to hypertonicity (300 mOsmol/L) for 48 h in juvenile stages of air-breathing stinging catfish (Heteropneustes fossilis). A total of 8 mRNA transcripts for different isoforms of AQPs and their translated proteins could be detected in the anterior and posterior regions of S1, S2, and S3 stages of juveniles of stinging catfish at variable levels. In general, more expression of mRNAs for different aqp genes was seen in the S2 and S3 juveniles than in the S1 juveniles. Most interestingly, exposure to hypertonicity of S2 juveniles for a period of 48 h led to increased expression of most of the aqp genes both at transcriptional and translational levels, except for aqp3 in the anterior and posterior regions and aqp1 in the anterior region, showing maximum expression at later stages of hypertonic exposure. Thus, it is evident that AQPs play crucial roles in maintaining the water and ionic balances under anisotonic conditions even at the early developmental stages of stinging catfish as a biochemical adaptational strategy to survive and grow in anisotonic environment.

Characterization of aquaporin-1ab (Aqp1ab) mRNA in mud loach (Misgurnus mizolepis) exposed to heavy metal and immunostimulant stimuli

Aquaporins (AQPs) are key proteins that regulate fluid homeostasis in cells via modulating osmotic water transport. In the present study, we identified three variants of Aqp1ab transcript (mmAQP1ab x1, mmAQP1ab x2, and mmAQP1ab x3) in mud loaches (Misgurnus mizolepis), and their expression patterns were examined in response to heavy metal and immunostimulant exposure. Mud loach Aqp1ab gene has a somewhat different organizational structure (i.e. five exons interrupted by four introns) compared to most other teleostean Aqp1ab orthologues, which have four exons. The 5'-flanking regulatory region of Aqp gene showed diverse transcription factor binding motifs, particularly those associated with stress/immune responses. Developmental expression patterns indicated that Aqp1ab mRNA was maternally inherited, presumably important for fine-tuning gene expression during embryonic and early larval developments. Expression of mud loach Aqp1ab mRNA was significantly and differentially modulated in several tissues (intestine, kidneys, spleen, and liver) in response to various heavy metal treatments. In addition, Aqp1ab gene expression was highly induced in response to immune challenge (LPS and polyI:C injections). Collectively, our results suggested that AQPs are multifunctional effectors playing diverse roles in cellular pathways relevant to immune and/or stress adaptation responses, in addition to their involvement in osmoregulation.

Molecular Characterization of Aquaporins Genes from the Razor Clam Sinonovacula constricta and Their Potential Role in Salinity Tolerance

Aquaporins (AQPs) play crucial roles in osmoregulation, but the knowledge about the functions of AQPs in Sinonovacula constricta is unclear. In this study, Sc-AQP1, Sc-AQP8, and Sc-AQP11 were identified from S. constricta, and the three Sc-AQPs are highly conserved compared to the known AQPs. The qRT-PCR analysis revealed that the highest mRNA expressions of Sc-AQP1, Sc-AQP8, and Sc-AQP11 were detected in the gill, digestive gland, and adductor muscle, respectively. In addition, the highest mRNA expression of Sc-AQP1 and Sc-AQP11 was detected in the D-shaped larvae stage, whereas that of SC-AQP8 was observed in the umbo larvae stage. The mRNA expression of Sc-AQP1, Sc-AQP8, and Sc-AQP11 significantly increased to 12.45-, 12.36-, and 27.44-folds post-exposure of low salinity (3.5 psu), while only Sc-AQP1 and Sc-AQP11 significantly increased post-exposure of high salinity (35 psu) (p < 0.01). The fluorescence in situ hybridization also showed that the salinity shift led to the boost of Sc-AQP1, Sc-AQP8, and Sc-AQP11 mRNA expression in gill filament, digestive gland, and adductor muscle, respectively. Knockdown of the Sc-AQP1 and Sc-AQP8 led to the decreased osmotic pressure in the hemolymph. Overall, these findings would contribute to the comprehension of the osmoregulation pattern of AQPs in S. constricta.

Deciphering the role of aquaporin 1 in the adaptation of the stinging catfish Heteropneustes fossilis to environmental hypertonicity by molecular dynamics simulation studies

A thorough investigation of the water permeability of H. fossilis aquaporin 1 (hfAQP1) in a hypertonic environment can provide a useful insight into the understanding of the underlying molecular mechanism of its high tolerance to salinity. Here, we constructed a 3 D homology model of hfAQP1 by taking Bos taurus AQP1, AQP0, and human AQP2 as templates using I-TASSER. The model obtained has similar structural organizations with mammalian AQP1s in all aspects. We investigated the water permeability of the modeled hfAQP1 in a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membrane under neutral and 100 mM hypersalinity by subjecting each system to a 100 ns molecular dynamics simulation. Our results show that hypersalinity hinders water permeation across the membrane through the hfAQP1 channel. A change in the intermolecular distance between key residues of the ar/R selectivity filter along with charge redistribution resulted in the accommodation of only 2-6 water molecules inside the channel at once under hypersaline conditions. We investigated the mRNA expression pattern of hfaqp1 in osmoregulatory organs of H. fossilis in response to 100 mM hypertonicity by using qPCR analysis. The transcript was downregulated in kidney and GI tract, but upregulated in the Gills. Thus, the catfish survive in a hypertonic environment by reducing the transport of water in its cellular systems and downregulating the expression of the hfaqp1 gene. The results observed in our study can shed more light on the functionality of AQP1 in catfishes under salinity stress and aid in future researches on solving more gating mechanisms involved in its regulation.Communicated by Ramaswamy H. Sarma.

Genomic and expression characterization of aquaporin genes from Siniperca chuatsi

Aquaporins (AQPs) are major intrinsic proteins that form pores in the membranes of biological cells. We first cloned the full-length sequences of aqp0, 1, 3, 4, 7, 8, 9, 10, 11, and 12 genes in Siniperca chuatsi. The 10 S. chuatsi aqp (Sc-aqp) genes included complete open reading frames and exhibited different exon-intron organizations. Sc-aqp1, 3, 8, 9, 10, and 11 were mostly expressed in the gallbladder, gills, gastric cecum, liver, ovaries, and spleen, respectively; Sc-aqp0 and 4 were mostly expressed in larvae at 1 day after hatching and in gastrula; Sc-aqp7 and 12 were mostly expressed in 2K-cell embryos. The expression levels of Sc-aqp1, 3, 7, 8, 9, and 10 after 10 part per thousand (ppt) salt treatment had significantly changed compared with those after 0 ppt salt treatment. Real-time quantitative PCR analysis further showed that in the intestines, the mRNA levels of Sc-aqp1 and 10 significantly decreased by approximately 2.07- and 2.85-fold, respectively, whereas those of Sc-aqp8 and 9 significantly increased by approximately 7.08- and 4.14-fold, respectively. Sc-aqp1, 8, 9, and 10 showed no significant differences in the gills. Sc-aqp3 significantly decreased by approximately 1.51- and 1.67-fold in the gills and intestines, respectively. Sc-aqp7 significantly increased by approximately 4.18- and 7.04-fold in the gills and intestines, respectively. This study was the first to investigate the tissue expression profiles and response to salt stress of aqp genes in S. chuatsi. Moreover, altering diet and suffering from immune stress could cause changes in the expression level of aqps. This study provided valuable reference information for AQPs' roles in osmoregulation in freshwater fish.

Interspecies Behavioral Variability of Medaka Fish Assessed by Comparative Phenomics

Recently, medaka has been used as a model organism in various research fields. However, even though it possesses several advantages over zebrafish, fewer studies were done in medaka compared to zebrafish, especially with regard to its behavior. Thus, to provide more information regarding its behavior and to demonstrate the behavioral differences between several species of medaka, we compared the behavioral performance and biomarker expression in the brain between four medaka fishes, Oryzias latipes, Oryzias dancena, Oryzias woworae, and Oryzias sinensis. We found that each medaka species explicitly exhibited different behaviors to each other, which might be related to the different basal levels of several biomarkers. Furthermore, by phenomics and genomic-based clustering, the differences between these medaka fishes were further investigated. Here, the phenomic-based clustering was based on the behavior results, while the genomic-based clustering was based on the sequence of the nd2 gene. As we expected, both clusterings showed some resemblances to each other in terms of the interspecies relationship between medaka and zebrafish. However, this similarity was not displayed by both clusterings in the medaka interspecies comparisons. Therefore, these results suggest a re-interpretation of several prior studies in comparative biology. We hope that these results contribute to the growing database of medaka fish phenotypes and provide one of the foundations for future phenomics studies of medaka fish.

Ion Transporters and Osmoregulation in the Kidney of Teleost Fishes as a Function of Salinity

Euryhaline teleosts exhibit major changes in renal function as they move between freshwater (FW) and seawater (SW) environments, thus tolerating large fluctuations in salinity. In FW, the kidney excretes large volumes of water through high glomerular filtration rates (GFR) and low tubular reabsorption rates, while actively reabsorbing most ions at high rates. The excreted product has a high urine flow rate (UFR) with a dilute composition. In SW, GFR is greatly reduced, and the tubules reabsorb as much water as possible, while actively secreting divalent ions. The excreted product has a low UFR, and is almost isosmotic to the blood plasma, with Mg²⁺, SO₄^2–, and Cl^– as the major ionic components. Early studies at the organismal level have described these basic patterns, while in the last two decades, studies of regulation at the cell and molecular level have been implemented, though only in a few euryhaline groups (salmonids, eels, tilapias, and fugus). There have been few studies combining the two approaches. The aim of the review is to integrate known aspects of renal physiology (reabsorption and secretion) with more recent advances in molecular water and solute physiology (gene and protein function of transporters). The renal transporters addressed include the subunits of the Na⁺, K⁺- ATPase (NKA) enzyme, monovalent ion transporters for Na⁺, Cl^–, and K⁺ (NKCC1, NKCC2, CLC-K, NCC, ROMK2), water transport pathways [aquaporins (AQP), claudins (CLDN)], and divalent ion transporters for SO₄^2–, Mg²⁺, and Ca²⁺ (SLC26A6, SLC26A1, SLC13A1, SLC41A1, CNNM2, CNNM3, NCX1, NCX2, PMCA). For each transport category, we address the current understanding at the molecular level, try to synthesize it with classical knowledge of overall renal function, and highlight knowledge gaps. Future research on the kidney of euryhaline fishes should focus on integrating changes in kidney reabsorption and secretion of ions with changes in transporter function at the cellular and molecular level (gene and protein verification) in different regions of the nephrons. An increased focus on the kidney individually and its functional integration with the other osmoregulatory organs (gills, skin and intestine) in maintaining overall homeostasis will have applied relevance for aquaculture.

Expression of aquaporins in response to distinct dehydration stresses that confer stress tolerance in the Antarctic midge Belgica antarctica

Larvae of the Antarctic midge Belgica antarctica Jacobs (Diptera: Chironomidae) are highly tolerant of diverse environmental stresses, including freezing, severe desiccation, and osmotic extremes. Furthermore, dehydration confers subsequent desiccation and freeze tolerance. While a role for aquaporins-channels for water and other solutes-has been proposed in these dehydration processes, the types of aquaporins involved in dehydration-driven stress tolerance remain unknown. In the present study, we investigated expression of six aquaporins (Drip, Prip, Eglp1, Eglp2, Aqp12L, and Bib) in larvae of B. antarctica subjected to three different dehydration conditions: desiccation, cryoprotective dehydration, and osmotic dehydration. The expression of Drip and Prip was up-regulated under desiccation and cryoprotective dehydration, suggesting a role for these aquaporins in efficient water loss under these dehydration conditions. Conversely, expression of Drip and Prip was down-regulated under osmotic dehydration, suggesting that their expression is suppressed in larvae to combat dehydration. Larval water content was similarly decreased under all three dehydration conditions. Differences in responses of the aquaporins to the three forms of dehydration suggests distinct water management strategies associated with different forms of dehydration stress.

Molecular assessment and transcriptome profiling of wild fish populations of Oryzias mekongensis and O. songkhramensis (Adrianichthyidae: Beloniformes) from Thailand

Among the fish of the genus Oryzias, two species are frequently used as model animals in biological research. In Thailand, Oryzias mekongensis is usually found in natural freshwater near the Mekong Basin in the northeast region, while O. songkhramensis inhabits the Songkhram Basin. For differential morphological identification, the coloured bands on the dorsal and ventral margins of the caudal fin are used to distinguish O. mekongensis from O. songkhramensis. However, these characteristics are insufficient to justify species differentiation, and little molecular evidence is available to supplement them. This study aimed to investigate the molecular population and transcriptome profiles of adult O. mekongensis and O. songkhramensis. In the molecular tree based on cytochrome b sequences, O. mekongensis exhibited four clades that were clearly distinguished from O. songkhramensis. Clade 1 of the O. mekongensis population was close to the Mekong River and lived in the eastern portion of the upper northeast region. Clade 2 was far from the Mekong River and inhabited the middle region of the Songkhram River. Clade 3 was positioned to the west of the Songkhram River, and clade 4 was to the south of the Songkhram River Basin. After RNA sequencing using an Illumina HiSeq 2500 platform, the gene category annotations hardly differentiated the species and were discussed in the text. Based on the present findings, population dispersal of these Oryzias species might be associated with geographic variations of the upper northeast region. Molecular genetics and transcriptome profiling might advance our understanding of the evolution of teleost fish.

Gene expression profiling of proximal and distal renal tubules in Atlantic salmon (Salmo salar) acclimated to fresh water and seawater

Euryhaline teleost kidneys undergo a major functional switch from being filtratory in freshwater (FW) to being predominantly secretory in seawater (SW) conditions. The transition involves both vascular and tubular effects. There is consensus that the glomerular filtration rate is greatly reduced upon exposure to hyperosmotic conditions. Yet, regulation at the tubular level has only been examined sporadically in a few different species. This study aimed to obtain a broader understanding of transcriptional regulation in proximal versus distal tubular segments during osmotic transitions. Proximal and distal tubule cells were dissected separately by laser capture microdissection, RNA was extracted, and relative mRNA expression levels of >30 targets involved in solute and water transport were quantified by quantitative PCR in relation to segment type in fish acclimated to FW or SW. The gene categories were aquaporins, solute transporters, fxyd proteins, and tight junction proteins. aqp8bb1, aqp10b1, nhe3, sglt1, slc41a1, cnnm3, fxyd12a, cldn3b, cldn10b, cldn15a, and cldn12 were expressed at a higher level in proximal compared with distal tubules. aqp1aa, aqp1ab, nka-a1a, nka-a1b, nkcc1a, nkcc2, ncc, clc-k, slc26a6C, sglt2, fxyd2, cldn3a, and occln were expressed at a higher level in distal compared with proximal tubules. Expression of aqp1aa, aqp3a1, aqp10b1, ncc, nhe3, cftr, sglt1, slc41a1, fxyd12a, cldn3a, cldn3b, cldn3c, cldn10b, cldn10e, cldn28a, and cldn30c was higher in SW- than in FW-acclimated salmon, whereas the opposite was the case for aqp1ab, slc26a6C, and fxyd2. The data show distinct segmental distribution of transport genes and a significant regulation of tubular transcripts when kidney function is modulated during salinity transitions.

Drinking and Water Handling in the Medaka Intestine: A Possible Role of Claudin-15 in Paracellular Absorption?

When euryhaline fish move between fresh water (FW) and seawater (SW), the intestine undergoes functional changes to handle imbibed SW. In Japanese medaka, the potential transcellular aquaporin-mediated conduits for water are paradoxically downregulated during SW acclimation, suggesting paracellular transport to be of principal importance in hyperosmotic conditions. In mammals, intestinal claudin-15 (CLDN15) forms paracellular channels for small cations and water, which may participate in water transport. Since two cldn15 paralogs, cldn15a and cldn15b, have previously been identified in medaka, we examined the salinity effects on their mRNA expression and immunolocalization in the intestine. In addition, we analyzed the drinking rate and intestinal water handling by adding non-absorbable radiotracers, 51-Cr-EDTA or 99-Tc-DTPA, to the water. The drinking rate was >2-fold higher in SW than FW-acclimated fish, and radiotracer experiments showed anterior accumulation in FW and posterior buildup in SW intestines. Salinity had no effect on expression of cldn15a, while cldn15b was approximately 100-fold higher in FW than SW. Despite differences in transcript dynamics, Cldn15a and Cldn15b proteins were both similarly localized in the apical tight junctions of enterocytes, co-localizing with occludin and with no apparent difference in localization and abundance between FW and SW. The stability of the Cldn15 protein suggests a physiological role in water transport in the medaka intestine.

Molecular analysis of population and De Novo transcriptome sequencing of Thai medaka, Oryzias minutillus (Teleostei: Adrianichthyidae)

Thai medaka (Oryzias minutillus) are alternatively known as Thai rice-fish or dwarf medaka, and they widely inhabit natural freshwater environments in all regions of Thailand. In this study, we aimed to investigate the molecular genetics of the Thai medaka population in Thailand inferred from the mitochondrial control region (D-loop) and the cytochrome c oxidase subunit 1 (coxI) sequences. Furthermore, we examined RNA sequencing (RNA-seq) of adult males and females was performed with next-generation sequencing. Together, the combination of the D-loop and coxI sequences clearly distinguished the Thai medaka populations into 2 groups, such as group 1, which generally included samples from the central, northern, western, and eastern regions of the northeastern region. In this group, the fish populations seem to be a little monophyly in which the first subpopulation comprised the main samples from the northern and central regions. The second subpopulation commonly contained fish from the eastern region and specimens from the southern part of the central region near the Gulf of Thailand. Although these subgroups related to geographical distribution, bootstrap values were low in branch considered significant for both subgroups. Group 2 consisted of almost all samples from the southern population and those from the central and southern part of the northeastern region. Group 2 was found that it was made of samples from the northeastern region and samples from the southern population. A total of 73551 unigenes were identified after gene annotation. Signal transduction was the predominant protein classification among the Thai medaka orthologous groups. A differentially expressed gene (DEG) analysis identified 6 subclusters between both sexes that were composed of 257, 131, 364, 386, 114 and 108 genes. Phototransduction was the most enriched pathway and was highly expressed in males, while viral carcinogenesis, oocyte genesis, and the complement and coagulation cascades were highly expressed in females. Further details of these DEGs are discussed below. These results suggest that Thai medaka may genetically exhibit independent populations in the geographic habitats of Thailand. Moreover, these fish also reveal the genes that are conserved in other organisms and those that may be specific to this species.

Differential Expression and Localization of Branchial AQP1 and AQP3 in Japanese Medaka (Oryzias latipes)

Aquaporins (AQPs) facilitate transmembrane water and solute transport, and in addition to contributing to transepithelial water transport, they safeguard cell volume homeostasis. This study examined the expression and localization of AQP1 and AQP3 in the gills of Japanese medaka (Oryzias latipes) in response to osmotic challenges and osmoregulatory hormones, cortisol, and prolactin (PRL). AQP3 mRNA was inversely regulated in response to salinity with high levels in ion-poor water (IPW), intermediate levels in freshwater (FW), and low levels in seawater (SW). AQP3 protein levels decreased upon SW acclimation. By comparison, AQP1 expression was unaffected by salinity. In ex vivo gill incubation experiments, AQP3 mRNA was stimulated by PRL in a time- and dose-dependent manner but was unaffected by cortisol. In contrast, AQP1 was unaffected by both PRL and cortisol. Confocal microscopy revealed that AQP3 was abundant in the periphery of gill filament epithelial cells and co-localized at low intensity with Na⁺,K⁺-ATPase in ionocytes. AQP1 was present at a very low intensity in most filament epithelial cells and red blood cells. No epithelial cells in the gill lamellae showed immunoreactivity to AQP3 or AQP1. We suggest that both AQPs contribute to cellular volume regulation in the gill epithelium and that AQP3 is particularly important under hypo-osmotic conditions, while expression of AQP1 is constitutive.

Molecular Characterization of Aquaporin 1 and Aquaporin 3 from the Gills of the African Lungfish, Protopterus annectens, and Changes in Their Branchial mRNA Expression Levels and Protein Abundance during Three Phases of Aestivation

African lungfishes can undergo long periods of aestivation on land during drought. During aestivation, lungfishes are confronted with desiccation and dehydration, and their gills become non-functional and covered with a thick layer of dried mucus. Aquaporins (Aqps) are a superfamily of integral membrane proteins which generally facilitate the permeation of water through plasma membranes. This study aimed to obtain the complete cDNA coding sequences of aqp1 and aqp3 from the gills of Protopterus annectens, and to determine their branchial mRNA and protein expression levels during the induction, maintenance and arousal phases of aestivation. Dendrogramic analyses of the deduced Aqp1 and Aqp3 amino acid sequences of P. annectens revealed their close relationships with those of Latimeria chalumnae and tetrapods. During the induction phase, there were significant decreases in the transcript levels of aqp1 and aqp3 in the gills of P. annectens, but the branchial Aqp1 and Aqp3 protein abundance remained unchanged. As changes in transcription might precede changes in translation, this could be regarded as an adaptive response to decrease the protein abundance of Aqp1 and Aqp3 in the subsequent maintenance phase of aestivation. As expected, the branchial transcript levels and protein abundance of aqp1/Aqp1 and aqp3/Aqp3 were significantly down-regulated during the maintenance phase, probably attributable to the shutdown of branchial functions and the cessation of volume regulation of branchial epithelial cells. Additionally, these changes could reduce the loss of water through branchial epithelial surfaces, supplementing the anti-desiccating property of the dried mucus. Upon arousal, it was essential for the lungfish to restore branchial functions. Indeed, the protein abundance of Aqp1 recovered partially, with complete recovery of mRNA expression level and protein abundance of Aqp3, in the gills of P. annectens after 3 days of arousal. These results provide insights into how P. annectens regulates branchial Aqp expression to cope with desiccation and rehydration during different phases of aestivation.

Aquaporin 1 Is Involved in Acid Secretion by Ionocytes of Zebrafish Embryos through Facilitating CO2 Transport

Mammalian aquaporin 1 (AQP1) is well known to function as a membrane channel for H₂O and CO₂ transport. Zebrafish AQP1a.1 (the homologue of mammalian AQP1) was recently identified in ionocytes of embryos; however its role in ionocytes is still unclear. In this study, we hypothesized that zebrafish AQP1a.1 is involved in the acid secretion by ionocytes through facilitating H₂O and CO₂ diffusion. A real-time PCR showed that mRNA levels of AQP1a.1 in embryos were induced by exposure to 1% CO₂ hypercapnia for 3 days. In situ hybridization and immunohistochemistry showed that the AQP1a.1 transcript was highly expressed by acid-secreting ionocytes, i.e., H⁺-ATPase-rich (HR) cells. A scanning ion-selective electrode technique (SIET) was applied to analyze CO₂-induced H⁺ secretion by individual ionocytes in embryos. H⁺ secretion by HR cells remarkably increased after a transient loading of CO₂ (1% for 10 min). AQP1a.1 knockdown with morpholino oligonucleotides decreased the H⁺ secretion of HR cells by about half and limited the CO₂ stimulated increase. In addition, exposure to an AQP inhibitor (PCMB) for 10 min also suppressed CO₂-induced H⁺ secretion. Results from this study support our hypothesis and provide in vivo evidence of the physiological role of AQP1 in CO₂ transport.

Water transport and functional dynamics of aquaporins in osmoregulatory organs of fishes

Aquaporins play distinct roles for water transport in fishes as they do in mammals-both at the cellular, organ, and organismal levels. However, with over 32,000 known species of fishes inhabiting almost every aquatic environment, from tidal pools, small mountain streams, to the oceans and extreme salty desert lakes, the challenge to obtain consensus as well as specific knowledge about aquaporin physiology in these vertebrate clades is overwhelming. Because the integumental surfaces of these animals are in intimate contact with the surrounding milieu, passive water loss and uptake represent two of the major osmoregulatory challenges that need compensation. However, neither obligatory nor regulatory water transport nor their mechanisms have been elucidated to the same degree as, for example, ion transport in fishes. Currently fewer than 60 papers address fish aquaporins. Most of these papers identify "what is present" and describe tissue expression patterns in various teleosts. The agnathans, chondrichthyans, and functionality of fish aquaporins generally have received little attention. This review emphasizes the functional physiology of aquaporins in fishes, focusing on transepithelial water transport in osmoregulatory organs in euryhaline species - primarily teleosts, but covering other taxonomic groups as well. Most current knowledge comes from teleosts, and there is a strong need for related information on older fish clades. Our survey aims to stimulate new, original research in this area and to bring together new collaborations across disciplines.

Cloning and characterization of aquaglyceroporin genes from rainbow smelt (Osmerus mordax) and transcript expression in response to cold temperature

Aquaglyceroporins (GLPs) are integral membrane proteins that facilitate passive movement of water, glycerol and urea across cellular membranes. In this study, GLP-encoding genes were characterized in rainbow smelt (Osmerus mordax mordax), an anadromous teleost that accumulates high glycerol and modest urea levels in plasma and tissues as an adaptive cryoprotectant mechanism in sub-zero temperatures. We report the gene and promoter sequences for two aqp10b paralogs (aqp10ba, aqp10bb) that are 82% identical at the predicted amino acid level, and aqp9b. Aqp10bb and aqp9b have the 6 exon structure common to vertebrate GLPs. Aqp10ba has 8 exons; there are two additional exons at the 5' end, and the promoter sequence is different from aqp10bb. Molecular phylogenetic analysis suggests that the aqp10b paralogs arose from a gene duplication event specific to the smelt lineage. Smelt GLP transcripts are ubiquitously expressed; however, aqp10ba transcripts were highest in kidney, aqp10bb transcripts were highest in kidney, intestine, pyloric caeca and brain, and aqp9b transcripts were highest in spleen, liver, red blood cells and kidney. In cold-temperature challenge experiments, plasma glycerol and urea levels were significantly higher in cold- compared to warm-acclimated smelt; however, GLP transcript levels were generally either significantly lower or remained constant. The exception was significantly higher aqp10ba transcript levels in kidney. High aqp10ba transcripts in smelt kidney that increase significantly in response to cold temperature in congruence with plasma urea suggest that this gene duplicate may have evolved to allow the re-absorption of urea to concomitantly conserve nitrogen and prevent freezing.

Aquaporin expression in the Japanese medaka (Oryzias latipes) in freshwater and seawater: challenging the paradigm of intestinal water transport?

We investigated the salinity-dependent expression dynamics of seven aquaporin paralogs (aqp1a, aqp3a, aqp7, aqp8ab, aqp10a, aqp10b and aqp11a) in several tissues of euryhaline Japanese medaka (Oryzias latipes). All paralogs except aqp7 and aqp10a had a broad tissue distribution, and several were affected by salinity in both osmoregulatory and non-osmoregulatory tissues. In the intestine, aqp1a, aqp7, aqp8ab and aqp10a decreased upon seawater (SW) acclimation in both long-term acclimated fish and during 1-3 days of the transition period. In the gill, aqp3a was lower and aqp10a higher in SW than in freshwater (FW). In the kidney no aqps were affected by salinity. In the skin, aqp1a and aqp3a were lower in SW than in FW. In the liver, aqp8ab and aqp10a were lower in SW than in FW. Furthermore, six Na(+),K(+)-ATPase α-subunit isoform transcripts were analysed in the intestine but none showed a consistent response to salinity, suggesting that water transport is not regulated at this level. In contrast, mRNA of the Na(+),K(+),2Cl(-)-cotransporter type-2 strongly increased in the intestine in SW compared with FW fish. Using custom-made antibodies, Aqp1a, Aqp8ab and Aqp10a were localized in the apical region of enterocytes of FW fish. Apical staining intensity strongly decreased, vanished or moved to subapical regions, when fish were acclimated to SW, supporting the lower mRNA expression in SW. Western blots confirmed the decrease in Aqp1a and Aqp10a in SW. The strong decrease in aquaporin expression in the intestine of SW fish is surprising, and challenges the paradigm for transepithelial intestinal water absorption in SW fishes.